Many electronic devices are configured as a hybrid of parts with different functions. In recent years, there are demands for downsizing and weight-saving these electronic devices from the perspective of mountability, and the efficient arrangement of the respective parts corresponding to the miniaturization of the substrate area or the enlargement of the active device are being demanded. Recently, thin film resistor elements, in particular, are used upon mounting these parts. The present invention relates to a metal foil with an electric resistance film in which a film with electric resistance is formed on a metal foil.
A copper foil is generally used as the wiring material of a printed circuit board. A copper foil is classified into an electrolytic copper foil and a rolled copper foil based on the production method thereof. The thickness of the copper foil can be arbitrarily adjusted from an extremely thin copper foil with a thickness of 5 μm to a thick copper foil with a thickness of approximately 140 μm.
These copper foils are bonded to a substrate made of resin such as epoxy or polyimide, and are used as a printed circuit board. Since a copper foil is required to possess sufficient adhesive strength for adhering to the resin as the substrate, with an electrolytic copper foil, its coarse surface referred to as the matte surface which is generally formed during the production thereof is used by being additionally subject to surface roughening treatment. With a rolled copper foil, its surface is also subject to roughening treatment prior to use.
In recent years, proposals have been made for additionally forming a thin film layer made of an electric resistance material on the copper foil as the wiring material (refer to Patent Documents 1 and 2). An electric resistance element is essential in an electronic circuit board, and, if a copper foil comprising a resistive layer is used, it is only necessary to expose the resistive element of the electric resistance film layer formed on the copper foil using an etching solution such as copper chloride. Thus, as a result of building the resistor element into the substrate, the limited surface area of the substrate can be used effectively compared to conventional methods of having to mount a chip resistive element on the surface of the substrate with solder bonding.
Moreover, design restrictions can be reduced by forming a resistive element in a multi-layered substrate, and the electrical properties can be improved since the circuit length can be shortened. Thus, if a copper foil comprising a resistive layer is used, solder bonding becomes unneeded or is considerably alleviated, and weight-saving can be achieved and reliability can be improved. As described above, a substrate with a built-in electric resistance film yields numerous advantages.
Nevertheless, conventional technology stopped at forming a specific electric resistance film on the metal foil, and there was no concept of forming various types of electric resistance films on the metal foil.    [Patent Document 1] Japanese Patent No. 3311338    [Patent Document 2] Japanese Patent No. 3452557